Electric temperature control



July 5, 1949. H. w. A. CHALBERG 2,475,309

ELECTRIC TEMPERATURE CONTROL Filed Feb; 20, 1947 Inventor; Harry W. A. Chalbergg,'

His Attorney.

Patented July 5, 1949 ELECTRIC TEMPERATURE CONTROL Harry W. A. Chalberg, Berne, N. Y., assignor to General Electric Company, a corporation of New York Application February 20, 1947, Serial No. 729,732

2 Claims.

The present invention relates to controls for electric heating devices and it has for its object the provision of improved means for controlling the energization of an electric heating device in a simple, reliable and efiicient manner.

More specifically, the present invention relates to an electronic temperature control and regulator for electric blankets. Electronic controls of the type employing an electric valve for controling the supply of energy to a blanket have heretoiore been proposed. In those applications, it has been proposed, for example to control the firing of an electric valve controlling the energization of the blanket by means of a resistance bridge network, one leg of which is a resistance located in the blanket so as to respond to its temperature. This leg is known as a feeler winding or element and is not a part of the blanket heating means itself, but is a separate element.

This invention also uses a resistance bridge network and a feeler element in the blanket, but does not use an electric valve of the discharge type. The present invention employs an emission limited diode to control the energization of a relay and thereby the energization of the blanket heater winding in response to the voltage output of the bridge network.

A more complete understanding of the invention will be obtained from the following description taken in connection with the accompanying drawing in which Fig. 1 is a representation of an electric blanket with a control device according to the present invention attached thereto and Fig. 2 is a schematic diagram of the heating device, in this case an electric blanket, and the control apparatus.

Fig. 1 illustrates an electric blanket I having attached thereto by suitable cable 2 a control box 3 having an external adjustable temperature selecting knob 4. A power input connection plug is illustrated at 5.

Referring now to Fig. 2, the electric blanket or other heating apparatus I is provided with a heating winding which may be conveniently made in two parts 6 and I connected in parallel. Where the device I is an electric blanket, the heating winding Ii, I will preferably be uniformly distributed within the blanket over substantially the entire blanket area; for example, a blanket body may be used like that described in Patent 2,203,918 to I. O. Moberg, issued June 11, 1940. In addition there is provided a ieeler winding 8 having a high temperature coefiicient of resistance and being preferably distributed and interwoven over the entire blanket area.

The feeler winding 8 and the heater winding 6, I may conveniently have a common terminal as at 9 and be connected by conductor II) to one side II of an electric power source, the other side of which is I2. In series with the other terminal of the feeler winding 8 there is a variable resistor I3 having an adjustable contact I4 controllable by means of the knob 4 shown in Fig. 1. This variable resistor enables the user of the apparatus to select the temperature to be produced by the blanket as will be explained later. The resistor I3 may be omitted where the temperature to be produced can be fixed at a predetermined degree.

The feeler winding 8 and variable resistor I3 together form one leg of a resistance bridge network. Two other legs of the bridge are formed by fixed resistor-s I5 and I6, each having one terminal connected to power supply conductor I2. The other terminal of resistor I5 is connected to variable resistor I3 while the remaining terminal of resistor I6 is connected to resistor I! which forms the fourth leg of the bridge. If desired, one of the other bridge resistors may be made variable, the resistor I3 in series with the feeler winding being omitted. The resistor I1 is preferably made of a material having a negative temperature coefiicient of resistance for the purpose of compensating the operation of the network for line voltage variations as will more clearly appear hereinafter. Power is applied across the bridge network with the resistances I6, I! in series, which combination is in parallel with the series resistances I5, I3, and 8.

The circuit further contains an emission limited diode I8 having filament I9, cathode 20 and anode 2I. The filament I9 is connected across the diagonal of the bridge opposite that to which the power supply is connected, namely, between the junction of resistors I B, I1 and junction of resistors I5, I3. With this arrangement it will be evident that the voltage applied to the diode filament I9 and consequently the filament current will depend upon the values of the bridge resistors. Thus, when the feeler winding 8 becomes hotter and its resistance increases, the current through filament I9 will decrease and vice versa. Also, when the contact I4 is moved to increase the effective resistance of resistor I3, the filament current will be decreased and vice versa. Similarly, a decrease in line voltage causing a decrease in current through resistor I'I results in an increase in the resistance of that element and a consequent decrease in the filament current and vice versa.

The diode I8 is an emission limited diode. By this I mean a vacuum tube whose plate current increases with increasing filament current and consequently increasing electron emission. The emission curve of such a diode, namely the graph of plate current against filament current will have an approximate straight line portion which is used herein as the operating region of the tube and in which the change in plate current is considerably greater than the change in filament current. In a true emission limited diode, the plate current is entirely controlled by the filament current and is not afiected by changes in line voltage. Insofar as this'ideal condition is not attainable in practice I provide compensating means as explained below.

Referring again to the drawing, the diode 18 has its cathode 20 connected to one side ll of the power supply and its anode 2i connected in series with the operating coil 22 of a relay 23 and plate resistor 24 to the other side l2 of the power supply. The relay 23, when the coil 22 is energized, serves to close the normally open contacts 25, one of which is connected by conductors 26 and 2'! to the heating windings 6, "i and the other of which is connected by conductor 28 to side [2 of the power supply. The contacts 25 therefore, when closed, connect the heating winding 6, 1 across the power supply. A condenser 29 is connected across relay coil 22 and resistor 24 to reduce the tendency of the relay to chatter when the power supply is alternating current, since the diode in this case acts as a half-wave rectifier.

There is further provided a resistance 3% connected in series with the heater winding 5, l in such a manner that this combination is in parallel with the feeler winding 8 when the rela contacts 25 are open. As shown in Fig. 2, one terminal of resistor 30 is connected to that relay contact which is connected to the heater winding 6, 1 while the other terminal is connected to the lower end of resistance l3. In operation, when power is applied to the system, assuming feeler winding 8 to be below the required temperature, the feeler winding will have a relatively low resistance, causing a relatively large voltage to be applied across the filament l9 and consequently a large plate current to flow through the diode l8 and relay winding 22, thereby closing the relay contacts 25 and connecting the heater winding 6, 1 across the line. After the feeler winding 8 has reached the required temperature, its resistance will be sufllciently high to have reduced the current through filament i9 and the plate current of the diode and consequently the current through relay coil- 22 to such an extent that the relay contacts 25 open, thereby disconnecting the heater winding 6, 1 from the line. When this occurs, however, resistance 3!] is in series with the heater winding to form a parallel circuit with the feeler element 8 in the term perature selector leg of the bridge. The eifective resistance of this leg of the bridge is thereby reduced resulting in an increase in filament current. On the other hand, after the resistance of feeler element 8 has reduced sufficiently to cause a.- sufficient increase in filament current tobring about a closing of the relay contacts, the resistor an is connected in parallel with resistor l5. Therefore, the effective resistance of this leg of the bridge is now reduced and there results a decrease in filament and plate currents, by a predetermined amount. The efliect of the resistor 39 upon the operation of the control circuit is therefore to reduce the temperature differential at the feeler element 8 which is required to cause the relay 23 to open and close.

Since the diode I 8 is an emission limited diode, that is one in which the plate current is substantially independent of plate voltage, the plate current will follow very sensitively all changes in filament current including those caused by line voltage variations. In order to obtain control circuit operation which is independent of line voltage variations, the bridge resistor I7 is, as previously stated, made of a material having a high negative temperature coefiicient of resistance. Variations in line voltage will, therefore, cause a variation in the current flowing through this resistor and its internal heating will produce a correspondingly opposite change in its resistance. By suitably enclosing the resistor I! while at the same time providing for sufficient dissipation of the heat generated within it, the voltage applied to the filament IQ of the diode can be maintained sufficiently independent of line voltage variations for all practical purposes.

While I have shown and described a particular embodiment of my invention, it will be obvious to those skilled in the art that various changes and modifications may be made without departing from my invention in its broader aspects and I, therefore, aim in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. An electric blanket control comprising a heating winding in the blanket, a feeler winding in the blanket adapted to change its resistance with change in its temperature, a resistance bridge network having said feeler winding as one leg, impedances external to the blanket forming the other three legs of said bridge, means supplying voltage across one pair of legs of said bridge, an emission limited diode having a filament connected across the opposite pair of legs of said bridge, whereby the filament current varies inversely as the. resistance of said feeler winding, relay means operated by the diode current for connecting and disconnecting said heating winding to and from a power source, and resistance means connected in the circuit so as to be in parallel with one of the bridge resistors when the heating winding is connected to the power source and in series with the heating winding to form a parallel circuit with the feeler winding when the heating winding is disconnected from the power source, whereby the temperature differential between operations of said relay means is reduced.

2. An electric blanket control comprising a heating winding in the blanket, a feeler winding in the blanket adapted to change its resistance with change in its temperature, a resistance bridge network having said feeler Winding as one leg, impedances external to the blanket forming the other three legs of said bridge, means supplying voltage across one pair of legs of said bridge, means connected across the opposite pair of legs of said bridge adapted to provide a current varying in response to voltage variations occurring across said opposite legs of the bridge, relay means operated by said current for connecting and disconnecting said heating winding to and from a power source, and resistance means connected in the circuit so as to be in parallel with one of the bridge resistors when the heating winding is connected to the power source and in series with the heating winding to form a parallel cir- 5 cuit with the feeler winding when the heating winding is disconnected from the power source, wherebythe temperature differential between operationsbf said relay means is reduced.

.HARRY W. A. CHALBECRG.

REFERENCES CITED The follewing references are of record in the file of this patent:

UNITED STATES PATENTS Name Date Storer May 21, 1907 Number Number Number Name Date Lyle Apr. 28, 1914 Respondek- Sept. 29, 1925 Hull Dec. 4, 1928 Gano June 4, 1929 Essex et a1 Sept. 30, 1930 Breisky Aug. 16, 1932 Donle et al Feb. 6, 1940 FOREIGN PATENTS Country Date Germany Dec. 24, 1935 

